Various electronic security systems are available for monitoring merchandise and discouraging theft from retail establishments. One system in widespread use consists of a transmitter for radiating a radio frequency field throughout a surveillance zone near the store exit; a tag, attached to the protected article, for receiving the transmitted signal and reradiating a signal having a different frequency or other detectable property; and a receiver for detecting the presence of the reradiated signal within the surveillance zone. An early example of such a system is disclosed in U.S. Pat. No. 3,493,955 to A. J. Minasy. The present inventor's U.S. Pat. Nos. 4,565,996 and 4,595,915 disclose improved rebroadcasting systems employing doppler techniques for establishing sharply defined surveillance zones thereby substantially reducing false alarms.
Experience has shown that although the mere presence of a tag on an article tends to deter theft, exit monitoring system do not provide total protection. For example, ways are found to remove the tags even if that results in damage to the merchandise. Such removal, of course, cannot be detected by exit control systems which can only sense the presence of tags within the surveillance zone.
Existing tag detection systems have other drawbacks. For example, although pilferers tend not to risk leaving a store with a tagged article, thefts of tagged items have occasionally succeeded by shielding the tag from the incident radiation while passing through the surveillance zone.
In addition, in clothing stores, monitoring tags are typically attached to garments by tacks which pierce the garment. (See, for example, U.S. Pat. No. 3,942,829 to Humble, et al.) Use of these fasteners can result in damage to the garment even during normal handling. Moreover, the removal of the tags at the checkout counter and their subsequent re-attachment to other garments can be time-consuming processes.
It is, of course, essential that the security system receiver respond only to signals from the tag and not to spurious signals from nearby sources such as electrical equipment, lighting fixtures or power lines subject to voltage changes. Yet despite elaborate efforts to eliminate them, false alarms continue to plague certain existing systems, especially those depending on the detection of amplitude modulated radio frequency signals.